LONDON, England (CNN) -- What to do with 50,000 used, possibly smelly and dirty prison mattresses that need to be disposed of every year?

Discarded mattresses pile up alongside trash in London in January of this year.

Britain's Prison Service won praise Thursday for its innovative solution to the problem: recycle the mattresses for carpet underlay, or break them down for fencing or even roof tiles.

Most of the mattresses are sent to landfills. There are enough of them each year to fill 30 double-decker buses, the National Audit Office said Thursday.

The Prison Service wanted to find a better way to dispose of the mattresses -- a "zero-waste" solution that also cost less money, the NAO said. It worked with suppliers and launched a competition to find the best solution.

They ended up coming up with several proposals to reuse the mattresses, and the Prison Service is now testing them out, the NAO said.

Thursday's report highlighted several other examples of ways that government departments have used innovation to their advantage.

"We have selected them as good examples of innovative approaches which have lessons for how innovation can be managed in government," the report said.